The present invention relates to testing equipment for determining strength characteristics of sheets. More particularly, the present invention relates to clamps for testing equipment, which clamps uniformly grip end portions of sheets to be loaded for testing of tensile or shear strength characteristics.
Elongate sheet materials are commonly used in mechanically stabilized earth retaining walls. The retaining walls are constructed with modular precast concrete members in the form of blocks or panels that stack on top of each other to create the vertical facing of the wall. The sheet materials extend laterally from connections with the blocks in the wall. The sheet materials are construction devices used to reinforce earthen slopes retained from slippage by the retaining wall, particularly where changes in elevations occur rapidly, for example, site developments with steeply rising embankments. These embankments must be secured against collapse or failure to protect persons and property from possible injury or damage caused by the slippage or sliding of the earthen slope.
Many designs for earth retaining walls exist today. Wall designs must account for lateral earth and water pressures, the weight of the wall, temperature and shrinkage effects, and earthquake loads. The design type known as mechanically stabilized earth retaining walls employ either metallic or polymeric tensile reinforcements in the soil mass. The tensile reinforcements extend laterally of the wall formed of facing units, typically precast concrete members, blocks, or panels stacked together. The tensile reinforcements connect the soil mass to the blocks that define the wall. The blocks create a visual vertical facing for the reinforced soil mass. The polymeric tensile reinforcements typically used are elongated lattice-like structures often referred to as grids. These are stiff polymeric extrusions. The grids have elongated ribs which connect to transversely aligned bars thereby forming elongated apertures between the ribs. Tensile reinforcements other than grids have been developed for use with mechanically stabilized earth retaining walls. These other tensile reinforcements are flexible reinforcement sheets, including large open-grid woven lattices and small aperture woven lattices, as well as woven textile sheets.
The specifications for earth retaining walls are based upon the strength of the interlocking components and the load created by the backfill. Once the desired wall height and type of ground conditions are known, the number of sheets, the vertical spacing between adjacent sheets, and lateral positioning of the sheets is determined, dependent upon the load capacity of the interlocking components.
To design an earth retaining wall, various strengths of the sheet material must be known in order to meet the specification for the site requirements. Sheet materials are tested to determine the tensile strength and also to test shear resistance to pullout. Tensile strength testing considers whether the sheet itself will fail by tearing. Pull-out resistance considers whether the sheet insufficiently engages the backfill material and thus slips laterally through the backfill. Testing of sheet materials is typically accomplished by independent labs. Design engineers use the test data to select the type and supplier of sheets for retaining wall projects.
During testing, at least one end of the sheet is secured by a clamp. There are a number of different types of clamps. Compression clamps secure the test sheet between two elongate members to which a compression loading is applied. Epoxy clamps use molding epoxy to form a build-up body around the test specimen. Bolts pass through the epoxy body for connecting to the test apparatus. Split wedge clamps apply a gripping force on the test sheet passing through a split wedge body. Roller grips wrap opposing portions of the test sheet around rollers.
For tensile strength testing, the opposing end of the sheet is secured by another clamp. Loading is then applied to the test sheet. A load cell measures the amount of force being applied to the test sheet. As the loading force continues to increase, the sheet ultimately fails. The tensile strength of the sheet is recorded.
Pullout testing examines the resistance of the sheet to pulling out from backfill material or from between the blocks in the wall. For backfill shear resistance testing, a portion of the test sheet is embedded with backfill material in a soil box. For normal load (block wall pullout), the sheet is placed between blocks used in constructing the wall. Loading is applied to move the clamp laterally. When the sheet pulls out from the soil box or from between the blocks, the shear resistance is recorded.
One drawback to some of these testing devices is that the loading is concentrated at the point of attachment. Accordingly, the failure of the sheet sometimes occurs at the point of attachment, rather than the failure occurring in an intermediate portion of the sheet. The testing therefore, does not provide a true measure of the tensile strength of the sheet material, but rather provides an indication of the strength at a concentrated point.
In some tests, the grip by the clamp on the test sheet slips. The higher the loading, the more likely the incidence of slippage in the clamp. This of course does not accurately test the sheet.
Also, clamps for some testing devices are time consuming to assemble and attach to the test sheet. Epoxy clamps particularly require a curing period, and the epoxy materials are single use only which increases costs. Split wedge clamps are awkward to assemble with the test sheet.
Accordingly, there is a need in the art for an improved clamp for use in testing apparatus for determining the tensile and pull-out resistance strengths of sheets. It is to such that the present invention is directed.
The present invention meets the need in the art by providing a holding clamp for sheet testing apparatus, which holding clamp engages an end portion of a reinforcement sheet to be tested for tensile or pull-out strengths. The holding clamp defines a channel having at least two adjacent bearing surfaces and an opening between the bearing surfaces to an exterior surface. The channel receives an elongate clamping bar that conforms in cross-sectional shape at least relative to the pair of adjacent bearing surfaces. An end portion of the test sheet wraps around the clamping bar, which is received in the channel with the sheet extending laterally through the opening. Depending on the testing, the opposing end of the sheet is secured by another holding device or by the backfill material or blocks. A loading device applies an increasing load to the sheet through the clamp for determining the strength of the sheet. The clamping bar mechanically engages the bearing surfaces of the channel to distribute the tensile loading to the bearing surfaces of the block.
In another aspect, the present invention provides a method of securing a reinforcement sheet with a clamp for use in a testing apparatus for determining a tensile or pull-out strengths of the reinforcement sheet, comprising the steps of:
(a) providing a holding clamp that defines a channel extending between opposing sides and having at least two adjacent bearing surfaces and an opening to an exterior surface thereof between the bearing surfaces;
(b) sliding a clamping bar overwrapped with an end portion of a sheet to be tested for tensile or pull-out strength along the channel with a portion of the reinforcement sheet extending outwardly through the opening, the clamping bar conforming in cross-sectional shape at least relative to the pair of adjacent bearing surfaces defined in the channel;
(c) securing an opposing end of the reinforcement sheet; and
(d) loading the reinforcement sheet through the clamp with an increasing force,
whereby the clamping bar, being wrapped by the reinforcement sheet that is loaded with the increasing force, mechanically engages the two bearing surfaces of the channel such that the tensile loading on the reinforcement sheet is applied to the two bearing surfaces in the block for measuring the strength of the reinforcement sheet.
Objects, advantages and features of the present invention will become apparent from a reading of the following detailed description of the invention and claims in view of the appended drawings.